1
DOI: 10.1002/ ((please add manuscript number))
Article type:
Bio-mimic four-dimensional printing of nanostructured interactive hydrogels
By Yufeng Tao
Yufeng Tao [email protected]
Chengchangfeng Lu
Whiting School of Engineering, Johns Hopkins University, Baltimore, 21218-2688 (USA)
Nature has exhibited numerous soft-interactive botanical or zoological systems with self-
adaptivenss or reconfigurable shape, however, artificial micro/nano materials of same merit are
formidable due to material/structural limitations, posing multidimensional challenges to current
chemical/physical fabrications. Here, we propose a new nanostructured hydrogel by four-
dimensional (4D) printing for function integration and dimension reduction. The cross-linked
matrix relaxes or contracts in robust hierarchical heterojunction to eliminate interface issues of
dual-layer desgin. Nanowire operates as minimum re-programmable unit (the smallest stimuli-
responsive hydrogel, volume less than one fiftieth of standard kirigami lattice). 4D hydrogels
promise solvent/light programmability, ultrafine feature size (<220 nm), large bending angle
(>1000°), variable Youngs’ modulus (>20 Mpa) and self-recovery ability. We fabricated
several homogeneous in/out-plane grippers, bio-mimic hands/muscles/actuators, soft scaffolds,
buckled microbridges and photon filters as verfication. Taking use of this maskless 4D printing,
many more programmable robotics/photonics devices far beyond demonstrations here could be
created.
Keywords: hierarchical heterojunction, hydrogel, laser chemistry, nanostructure, shape
reconfiguration.
1. Introduction
Four-dimensional (4D) printing renders permanent structures with time-participated shape
programmability, [1] opening up a new scientific frontier with unimaginable potential in material
design, prototyping, manufacturing and assembly.[2] The shape-morphable hydrogels are one of
2
pioneering 4D products (artificial grippers/robotics,[3] deployable cages/scaffolds,[4] drug
delivery, [5] tissue regeneration, [6] optics/photonics, [7] soft machines, [8] and strain sensor). [9]
However, current-stage hydrogels have yet to achieve perfection due to isotropic swelling or
shrinkage, [6, 7, 10] slow water diffusivity (<10-9 m2s-1), [11] inadequate stiffness and poor function.
Microcosmic environments (the blood vessel, vein, artery, skin pore, microfluidics or on-chip
labs) are physically inaccessible for macro hydrogels.[3-11] Thus, reducing dimension of
functional hydrogel become a prerequisite for integrating robotic applications. [12,13]
The last decade witnesses an explosive application of two-photon polymerization (TPP)
across a broad range of mechanics/bio-science/optics devices. [14,15] This predominant real-3D
microstructuring tool [16] generally produces inanimate sculptures deprived of time-depending
behavior. [17] Therefore, researchers often fabricated compound bilayer/trilayer structures for
controllable stimuli-responsiveness. [18,19] Unfortunately, bilayers require tedious multi-step
multi-material (at least active and inert materials) process, often transit one-way if releasing out
residual stress, and suffer from poor interfacial dis-matching.[18,20-22] Till now, multi-functional
hydrogels via homogenous material in rapid manner remains challenging but highly-valued.[21]
In nature, plants/animals utilizes water sorption-to-desorption for directional motion or
shape morphing.[23] Living organisms of hard-soft arranged hierarchical architectures wriggle
to transport in/out substances. Pinecones self-open/close according to drying/humidifying. Sea
cucumbers stiffen or soften by vomiting/taking water. Mimosa shut by directional movement
of water molecules on upper and lower surface of leaf pillow. Tendrils and bracts varies internal
turgor by applied stimuli. Natural materials such as fish scales, spider silk or nacre demonstrated
outstanding shape-changing properties for survival under external shock. Butterfly wings or
chameleon skin reflected multiplex structural colors. [24]
3
To overcome technological bottlenecks and integrate bio-mimic functions on-chip, [25] we
propose a covalently-interconnected heterojunction to TPP. Amphiphilic polyethylene glycol
diacrylate (PEG-DA) [26] was photon-incorporated with N-isopropylacrylamide (NIPAM) [10] at
high-order path modulation, obtaining mechanics-tunable nanostructured interactive hydrogels
(MNIHs, Figures 1, S1-S3, Video 1). Following that, we demonstrated in/out-plane interactive
grippers, tunable photon filters, bio-mimic hands/muscles (Figure 1a), light-fuelled actuators
showcasing some tempting benefits to science commnities:
1. The nanowire (NW, linewidth <100 nm, height <800 nm if using 100×oiled objective)
operated as minimum programming unit (the smallest hydrogel, at least two-magnitude smaller
than standard origami), guaranteeing ultrafine resolution in spatiotemporal reconfigurations.[27]
2. Self-driven mechanism was a task taken by material scicence for pushing frontier of
robotics. Here, intermolecular hydrogen bonding affected net-point number, making MNIHs
self-control shape without organs/generators or sensing components.
3. MNIHs contained foldable volume of loosely-linked intact network (Figure 1b) for self-
recovery if undergoing unwanted deformation, which improved longevity of devices, and
decreased repairing cost/complexity.
4. Rapid fabrication provided homogenous hydrogels, no interaction issue existed like
dual-layer devices, MNIHs maintained desirable structural stability in reverse reconfiguration.
More merits such as high fatigue resistance, large force-to-weight ratio, predictable high-
freedom programmability (Figure 1c) and desirable optical clarity could be found in this work.
4
Figure 1. (a) Schematic of femtosecond laser processing interactive materials for devising
multi-functional MNIHs. (b) Experiment optical system and laser manipulating NWs to control
intermolecular force and mechanical compliance. Length, direction and density of NWs jointly
determine shape programming. (c) SEM images of a cantilever self-standing on substrate by
shape programmability and qualitative morphological computation by finite element modelling
to predict morphing trend and stress distribution.
2. Results
Cationic phenothiazine, methylthionine chloride (MC) contained nitro and sulfur groups
as electron acceptors for higher-probability photon absorption (doping concentration as low as
0.5 wt%). [28,29] The prepared photoresist demonstrated suitable adhesion/stress/mobility on
substrates during single-step fabrication of various 3D architectures (Figures 1a, S1 and Video
1). Detailed chemical reaction was analyzed by Fourier transform infrared spectroscopy (FT-
IR, seen in Figures S4). [30] Concomitant photon-excited fluorescence highlighted near-infrared
beam for positioning or high-fidelity imaging. Laser travelling route implanted MNIHs with
initial geometric shape memory (Figure S1). MNIHs converted osmotic chemicals to kinetic
energy, [31] where functional groups of PEG-DA/NIPAM activate intact network to perceive
external stimuli or desorb matters to introduce nonlinear mechanical behaviors (Video 1). [32]
5
Transformative helix
Helix (or “spirochete”) denoted a typical structure in robotic swimmers or holders. [1-4,12,19]
Here, we fistly constructed 3D “bracelet” (Figure 2) at a horizontal expansion > 120% and a
lateral expansion < 8% from a planar (dimensions of 30×150×4 μm3, scanning speed of 1 mm/s,
costing 50 sec). Then, we intentionally-titling NWs to steer bending direction, MNIH self-
buckled, folded, rolled and rotated forcefully at least three turns (β > 1000°, tilting angle equaled
30o, Figures 1b and 2b), forming an out-plane DNA-like “spirochete”, or shrunk, opened, lied
down to starter shape (Video 2). No any breaking-up or collapsing was found in repeated
reconfiguration.
By carefully determining position of NWs, the shape morphing could occur throughout
the entire hydrogel, or just locally occurred in specific areas. Multiple 3D forms self-established
from planar starting shape, triggered by single droplet of solvents/oils. We regulated local duty
cycle for multi-point buckling (Figure S5). Individual “arch bridges” spontaneously stood at
specific coordinates, regions of bigger ΔL buckled out-plane as cross-sectional view evidenced
by scanning laser confocal microscopy (SLCM, Figure S5). Experiment results revealed the
bending angle β depended on trade-off balanced by line pitch ΔL and NWs’ width ΔN.
In cycles of interactive transformation, cross-linked network ensured tunable mechanical
properties and desirable structrual integrity during nonlinear reconfiguraiton. [33, 34] A small
deviation on rotating angle of less than 5% (Figure 2b) satified most of actuation requirements.
The nanostructured heterjunction exhibited highly-directional bending, twisting, rotating and
buckling-up with desirable repeatability (no observable fatigue phenomenon after 1K time
transformation), which took less responsive time than macro hydrogels, accelerated device
design-to-prototype process, and allowed multiple geometries from same starter shape.
6
Figure 2. (a) Accumulation of intermolecular force realized pre-designed responsiveness.
Constant-ΔL MNIH transformed to a “C” bracelet. (b) Titling-NW MNIH crimped into 3D
helix at three turns reversely. (c) Repeated transformation confirmed a large bendable angle
exceeding 1080°. (d) Experiment setup for on-chip generating structural coloration from wide-
spectrum white light source, and on-chip PC arrays. (e) Shape-deformable PC array, brilliant
coloration red-shifted and broadened through self-assembly, zoomed-in SEM images of NWs
before and after swelling.
Reconfigurable photonic crystal
4D-printed hydrogel nanowires synergized shape-morphing capability and high-resolution
nanostructure for nano-photonics (photonic crystal, PC), allowing PC to dynamically effect optical
field.[7] We developed on-chip interactive photonic crystals consisted of NWs to filter out
different-frequency photons as chip-integrated light sources, memories or displaying. [24] A full-
7
color chip reflected broad-band structural colors (purple, blue, green, yellow, orange, red and
their transitional-wavelength color in Figures 1a and 2d) by carefully modifying the scanning
path and optics parameters (Figure S7).
Temperature/solvents transformed periodic structural distribution and dielectric constants
to modulate photonic bandgap, [35] which shifted and broadened optical frequency for photonic
engineering. Exampled by on-chip generation of coloration via single-layer PC as photon filters
(size of 240×240 μm2 for each), wavelength-controllable highlighted structural colors irradiated
from a small area of single chip. PCs could be immersed in water to hide information, or heated/
humidified to show structural colors filtered from a white-light source (Figure 2d), as dynamic
labels used in encode information and anti-counterfeiting. [24, 35] Self-assembling PEG-400
modified NWs’ morphology and changed their dielectric constant difference, therefore,
structural coloration red-shifted and broadened (Figure 2e). Using our proposed interactive
material formula and 4D printing techniques, all researchers could easily design, tuning and
modelling their dynamic hydrogel photonics for numerous programmable optical components
(grating, modulator, coupler or display plane) of simultaneous shape/light manipulation, or
meta photonics. [35]
Bio-mimic hand and grippers
Reusable high-freedom holding or gripping was fundamental manipulation in microfluidic,
robotics or bio-sciences. [2-5, 25] MNIHs promised various in/out-plane muscle behaviors for
satisfying this aim (Figure 3). Exampled by a five-fingered dexterous palm precursor (Figure
3a), its thumb bent upward, and index/middle/ring/little fingers gradually clenched into a fist.
Similarly, closure of apricot flower-mimic MNIH (Figure 3b) and a cross-pattern gripper
(Figure 3d) were programmed, we theoretically-reconstruct the shape reconfigurations in the
frame of an explicit finite element method (Video 3).
8
Far beyond simply programming shape-changing direction, bending shape and triggering
condition of arms became programmable by arranging different heterojunctions. Here, an in-
plane gripping MNIH was designed to change its C-arm to S-arm (Figures 3c and S6, Video 3),
simultaneously, the inspiring condition switched oppositely. Water made C-arm closed, but S-
arm opened. All in/out-plane grippers self-opened or closed (Figure S6) in absence of built-in
electric/pneumatic drivers for implanting robots of different situations. [12, 25] We programmed
a swellable woodpile (dimension of 80×80×30 μm3, contracted in air and expanded by diluted
blood as cage machines, potentially assisting to trap cells/microorganism, Figure S6, Video 4 ).
Figure 3. Harnessing the temperature-triggered shape deformation provided great interest to
emerging micro-mechanic/bio-science fields and was identified here. (a) A hand-mimic MNIH
gradually curled into a clenched fist. (b) A heat-triggered flower-mimic closure. (c) Models and
SEM images of in-plane S-shape arm of soft-interactive grippers, and the traditional C-shape
arm. (d) A cross-shaped out-plane gripper and top view of finite element simulation. (e) A frog-
mimic MNIH in water for non-contact laser driven actuation, SEM images, and actuated shape
change due to laser point.
Laser-driven actuator
9
Outperforming local/entire shape programmability, MNIHs supported more useful 4D
behavior, the widely-concerned reverse light-deformable ability.[36] The unique NIPAM/PEG-
DA network possessed intrinsic photon-thermal conversion ratio without doping single-walled
carbon nanotube or graphene. Significantly different to solvent diffusion process, light-driven
thermo-responsive behaviors replied on no solvent/chemicals. [37,38] The laser-projected region
of MNIH generated local stress variation and expanded for pointed shape morphing, therefore,
an external laser can remotely trigger micro-scale actuation.
A free-standing frog-mimic MNIH was fabricated by 4D nano printing and put in water
for swelling (Figure 3e). Subsequently, we laser-scanned this fog externally, the photon-thermal
conversion redistribute the local stress inside MNIHs, therefore, frog became living and nodded
tirelessly (Video 5). When laser-point was remove, frog recover to stationary gesture. Long-
term results confirmed that nodding frequency and amplitude strongly depended on laser
scanning speed and optical power, no fatigue was found after at least 10000 times actuation.
Self recovery
Another competitive advantage, MNIHs promised intrinsic impact/shock-absorbing ability
outperforming permanent sculptures (covalent bond-formed structures broke into waste if
deformed). [2,3] Even being heated, distorted or squeezed by externally-applied force, MNIHs
not disintegrated but self-recovered to shape memory by absorbing organic matters (< 0.1 mL,
Figure 4), implying a remarkable survival ability in extreme conditions. We in-situ fixed a
“broken heart” (dimension of 500×700×15 μm3) distorted by a spun metal tweezer beforehand
to pre-designed symmetrical shape (recover rate exceeded 90%, Video 6), the cavitation-
induced air bubbles escaped away.
Mobile hydrophilic supra-molecules worked as nano-binders for regeneration in defected
morphologies. [9] Strength of hydrogen bonding depended on surface energy, interfacial
condition, electrostatic attraction force and volatility of materials. [9,15] Following that, we poked
10
a woodpile MNIH using a sharp needle, leaving a collapsed hole, then dropping PEG-400 as
nano-binder, woodpile absorbed PEG-400 molecules and apparently repaired holes (Figure 4c,
Video 6). Mechanical testing shown the repaired hole region reached over 70% Young’s
modulus (12MPa) of normal state (15 MPa, Figure 4d), and initial feature get recovered.
Figure 4. MNIHs got repairing by self-assembly according to shape memory. (a) An squeezed
malformed “heart” self-recovered to initial symmetric heart. (b) Illustration of self-assembling
molecules in fissures. (c) A needle-poked hole self-reconstructed by absorbing PEG-400, which
exhibited a new on-line repairing method for on-chip functional devices. (d) Schematic of
mechanical test, and Youngs’ modulus of initial and recovered MNIHs. (e) Shrinkage of
MNIHs (Video 7), weight-loss curve and its derivative in TGA test. Cross-linked hydrogel (blue
color) presents substantial weight loss around LCSTNIPAM. Pure PEG-DA-based, polymeric-
tested sample showed little temperature sensitivity within a zone of 30-45 oC.
Solvent retention
Self assembly of hydrogel was mirrored by solvent retention among MNIHs.[39] Reswelled
MNIHs were taken thermogravimetric analysis (TGA) for analysis solvent retention ability.
11
Results shown relative weight loss varied differently with or without extender NIPAM (43%
and 36%, respectively Figure 4e). NIPAM monomer transited hydrophilicity/hydrophobicity at
lower critical solution temperature (LCSTNIPAM=32.4 oC), and PEG-DA presented monotonous
hydrophility below 100 oC (Figure S2). 4D nano-printing changed density and thermal
conductivity of chain-style polymerized PNIPAM, making hydrophobic behavior occurred
within a broadened 30~42 oC zone. Seen in measured results, the hybridized NIPAM/PEG-DA
matrix decreased over 8% weight around LCSTNIPAM, while pure PEG-DA smoothly lost only
2.1% (Figure 4e).
Nanoscale morphologies
To identify nanoscale morphologies of swelling and shrinkage, we post-processed MNIHs
through freezing-to-drying for nanometric morphologies, SEM images presented an interlaced,
folded, nanofibrous-like long-chain conformation rather than porous morphology of pure
PNIPAM (Figure 5), and sustained a big interfacial contact area for mobility improvement.
Local molecular conformation switched for folding and unfolding states (Figure 5a), which
effected the density/mechanics of MNIHs significantly.
Mechanical properties
Superior virtue of both stiff and soft mechanical property was pivotal to address challenges
of biological interconnects, biomedical devices, soft machines/robots and facility.[34,40] Young’s
modulus (E) of stimuli-responsive MNIHs exhibited a wide MPa variation for generating tensile
force. We conducted an in situ force-sensing probe of a micromechanics platform to penetrate
into (compressive) and pull out (tensile) from MNIHs for stiffness (Figure 5b). Synchronous
stress (load force) and displacement were recorded and data-fitted for E (Figure 5c), an elastic
curve represented a hysteresis loop, confirming MNIHs as mechanics-tunable material. [40]
12
Figure 5. (a) Cantilever MNIH was equilibrium-swelled, subsequently freezed at -5 oC for 4 h
and vacuum-dried (DZF-6020, Boxun) at 40 oC for SEM characterization. (b) Hysteresis loop
of stress-to-displacement data (stiffness) in loading and un-loading processes, where post-bake,
drying, MNIH, and in-water refer to the 100 oC baked MNIH, 30 oC-dehydrated MNIH, the
just-developed MNIH, and the water-immersed MNIH. (c) The summarized Young’s modulus
of MNIHs at ΔL/ΔN (Spacing/NW width) ratio. (d) Different swelling performance of available
osmotic organic matters (DMF, DMSO, IPA and NMP refer to dimethylformamide, dimethyl
sulfoxide, isopropanol, and N-methyl pyrrolidone respectively). (e) Dynamic water contact
angle measured on surface of different material ratio MNIHs.
MNIHs achieved a standard tunable MPa level Eo of 18 ± 6 MPa (by changing spacing and
laser exposure dose, where the Poisson’s ratio ν = 0.49), [41] which decreased to 4~13 MPa
immersed in water, increased to 21~34 MPa if evaporation occurred, and jumped to 65 ± 8 MPa
after 100 oC post-bake (Figure 5c), which was stiffer than elastomer poly dimethylsiloxane
13
(PDMS) (EPDMS was 5~20 MPa) or Ecoflex™, but softer than poly methyl methacrylate (PMMA)
(EPMMA > 1 GPa).
Stimuli materials
In addition of the existing stimuli (temperature, pH, light, ion and water), our experiment
confirmed more available osmotic organic matters (dimethylformamide, dimethyl sulfoxide,
isopropanol, N-methylpyrrolidone, styrene sulfonate, methanol and glycol, Figure 5d), tissue
liquids (sweat, eye tears, blood, saliva and even urine) and even edible oils can drive MNIHs
out of equilibrium for shape reconfiguration. Acetone-type solvents also can trigger MNIHs in
a few of time but with destructive solubility.
MNIH automatically investigate stimuli without sensing components (in stark contrast to
sensor-embedded robot), which would escalate the interest of sensory materials. Huge impact
on responsivity could be created by altering photoresist formula, exposure dose or ΔL created
a huge impact on responsivity (Figure S), as summarized, water-contact angle (Figure 5e)
declined 63.35°56.49°46.25°37.87° on order of NIPAM:PEG-DA ratio increased
0.5:11:13:15:1. Hydrophilic/oleophylic MNIHs self-assembly organic solvents/oils for
organized deployment, potentially revolutionize therapeutic and diagnostic procedure triggered
by tissue liquids that was not possible otherwise.
Self-driven mechanism
Trick of the new self-driven mechanism lied on manipulating functional groups density to
accumulate intermolecular-based reconfigurable stress. As bi-functional material, MNIHs
generated negative angular momentum in swelling, driving them to an “n” or “c” shape,
however, thermal shrinkage generated positive angular momentum to lift upward as a “u” or
“L” shape. If the induced tension defeated adhesive constraint, out-plane actuation occurred.
Otherwise, MNIHs underwent in-plane shape morphing. To empirically explain shape
14
reconfiguration, a theoretical model delineated the relationship between the Young’s modulus
E and surface tensile force ΔF. Seen in Equation 2 (I = moment of inertia, unit: Kg•m2; ρ =
radius of curvature in the beam deflection theory, unit: μm-1), declining -ΔE created a negative
bending moment ΔM (unit: N•m) over the rectangular hydrogel sheet. Surface tensile force ΔF
resulted from integration calculation of local residual stress, Δσ, (unit:N) on single-layer
thickness H through entire length L (Equations 3 and 4, ε = local strain, ν = Poisson ratio of the
fabricated gelation).
∆𝐿 = 𝑘∆𝑁, 𝑘 ∈ [0.5 2.5] (1)
∆𝑀 = −∆𝐸𝐼 𝜌⁄ (2)
∆𝜎 = −∆𝐸𝜀 (1 − 𝑣)⁄ (3)
∆F = ∫ 𝐿∆𝜎𝑑𝑧𝐻
0 (4)
Dynamic Em was data fitted by averaging stiffness of ΔS/ΔD in penetrating and pulling-
out processes as Equation 5:
𝐸𝑚=1/2m 𝑃𝑤𝑇𝑡(∆S+/∆D++∆S-/∆D-). (5)
where m, Pw , Tt denoted coefficients of stiffness-to-Young’s modulus, effective laser power
and interaction time, bending angles followed below exponential relationship (Equations 6 and
7). Angle was β(t), t denoted time, τ was a time constant determined by material, and L denotes
the lateral bending length. The observed curvature, κ, reached 1 µm-1 (Figure S6e).
𝛽(𝑡) = 𝛽𝑜[1 − 𝑒𝑥𝑝 (𝑡/𝜏)] (6)
𝑘 = 𝛽/𝐿 (7)
3. Conclusion
4D nano printing cooperated with interactive composite materials to unleash capacity of
hydrogels breaking constraints of traditional TPP, which reduced dimension, shortened design-
to-prototype period, improved shape programmability. Functional groups governed self-
assembly for local or throughout softness-to-stiffness change. MNIHs self-interact with osmotic
chemicals, light or temperature to store/release mechanical energy, where cross-linked long-
chain conformation worked as neuromorphic matrix. Here resolution was selective (Figure S7),
defect was self-repairable, mechanic properties were reversible tunable (4~30 MPa), time and
15
material cost was saved. Moreover, intergrating individual micro-machines/nano-
photonic/bionic device by our proposed method promised an unimaginable potential of holistic
systematic applications.
4. Experimental Section
Material preparation
All ingredients were available from Sigma-Aldrich. We prepared glycol ((CH2OH)2, 0.3
mL, 99+% purity, molecular weight Mw = 62.068, solvent), NIPAM (C6H11NO, 0.6696 mg,
99.99+% purity, Mw = 113.16, extender), PEG-DA ((C3H3O).(C2H4O)n.(C3H3O), 1.8 ml,
99.999+% purity, Mw = 400-700, cross-linker) and methylthionline chloride (MC,
C16H18ClN3S, 99+% purity, photon initiator). The mixture of PEG-DA/NIPAM was added to
potassium persulfate (K2S2O8, 0.001 mg, Mw = 270.32) for pre-polymerization at room
temperature. Subsequently, more NIPAM (0.3348 mg), PEG-DA (0.9 mL) and MC (0.03 mg)
joined the pre-polymerized mixture under magnetic stirring at 800 rpm for 1h. Photoresist was
centrifugation purified by removing sediment before usage (Figures S2 and S3).
Morphology characterization
Substrates were pre-coated with conductive indium tin oxide (ITO) (100 nm thickness)
film for electric conductivity. Nanoscale characterization replied on the field-emission electron
microscope (FEI Nova NanoSEM™ 450) at acceleration voltages of 2-10 kV, magnifying 100-
30,000 times, or an ESEM (Thermo Scientific™ Quanta™ 200, FEI) on low-vacuum condition
for secondary-electron images. All MNIHs were freezing-to-drying processed before SEM for
more details. We also utilized scanning laser confocal microscope (LEXT OLS5000™,
Olympus) to nondestructively evaluate surface profile at subwavelength accuracy.
Laser direct writing
A mode-locked femtosecond Ti:Sapphire laser (Chameleon Discovery, Coherent) emitted
a wavelength-tunable, pulsed laser beam (80 MHz repetition rate, 100 fs pulse width) to initiate
16
a cross-linking reaction (power density of 2 to 20 mW/µm-2, exposure time of 0.4 to 8 ms).
Available laser wavelength covered visible and invisible ranges (754 to 956 nm). A terminal
microscope contained a charge-coupled device (CCD), dichroic mirror, and objective-selective
system. A close-looped, nano-step piezo displacement stage (300 × 300 × 300 µm3 range and
0.2 nm step size) moved in predesigned 3D trajectories (Figures S1 and S7).
Thermogravimetric analysis
We conducted thermogravimetric analysis (TGA) to real-time monitor changeable solvent
retention versus temperature. Equilibrium-swelled MNIHs underwent water evaporation when
ambient temperature ramped up at 5 oC/min by a thermogravimetric analyzer (Q500, TA
Instruments, resolution of 0.1 μg) within low temperature zone (30 ~ 100 oC).
Optical spectroscopy/imaging
To elucidate photon-chemical reaction during hydrogelation, Fourier transform infrared
spectroscopy of powders (NIPAM, MC), liquid (PEG-DA), and MNIHs were performed by a
Fourier Transform Infrared Spectrometer (FT-IR) (Nicolet™ Nexus 670, Thermo Scientific™)
at wavenumbers of 500 to 4000 cm-1 to deduct molecular structural transferring (Figure S4).
In situ micromechanical measurements
MNIH possessed magnificent mechanical programming properties that could be harnessed
for dynamic applications. Herein, an advanced micromechanics testing instrument (FT-MTA02,
FemtoTools) was deployed to investigate MNIH’s stiffness, cohesive behavior and output force
amplitude. The tungsten probes (2 µm tip radius) contained an in-packaged capacitive force
sensor to reflect mechanics during cycles of loading/unlading process at 5 nN resolution in 1000
µF range. Heating operation was realized by a thermoelectric cooler at 0.625 oC resolution
within -20~80 oC range or galvanometric laser-scanned MNIHs for photo-thermol conversation.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
17
Acknowledgements
This research was financially supported by the National Key R&D Program of China
(2017YFB1104300), National Natural Science Foundation of China (61774067), National
Science Foundation (CMMI 1265122), National Science Youth Fund of China (61805094), and
the Fundamental Research Funds for the Central Universities (HUST:2018KFYXKJC027).
Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
References
[1] F. Momeni, S. Mehdi Hassani.N, X. Liu, J. Ni. A review of 4D printing, Mater. Design.
2017, 122, 42–79
[2] S.D. Miao, N. Castro, M. Nowicki, L. Xia, H.T. Cui, X. Zhou, W. Zhu, S. Lee, K. Sarkar,
G. Vozzi, Y. Tabata, J. Fiser, L. G. Zhang, 4D printing of polymeric materials for tissue and
organ regeneration, Mater. Today. 2017, 20(10), 577-591
[3] I. Apsite, A. Biswas, Y. Li, L. Ionov, Microfabrication using shape-transforming materials,
Adv. Funct. Mater. 2020, 1908028
[4] Y. Hu, Z. Wang, D. Jin, C. Zhang, R. Sun, Z. Li, K. Hu, J. Ni, Z. Cai, D. Pan, X. Wang, W.
Zhu, J. Li, D. Wu, L. Zhang, J. Chu. Botanical‐inspired 4D printing of hydrogel at the
microscale, Adv. Funct. Mater. 2019, DOI:10.1002/adfm.201907377
[5] M.Y. Shie, Y.F. Shen, S. D. Astuti, A. K. Lee, S. H. Lin, N. Dwijaksara, and Y.W. Chen
Review of Polymeric Materials in 4D Printing Biomedical Applications, Polymers. 2019,
11(11):1864
[6] A.S. Gladman, E.A. Matsumoto, R.G. Nuzzo, L. Mahadevan, J.A. Lewis, Biomimetic 4D
printing, Nat. Mater. 2016, 15, 413–418
[7]H. Y. Jeong, E. Lee, S. An, Y. Lim, Y. C. Jun, 3D and 4D printing for optics and
metaphotonics, Nanophotonics, 2020, DOI: 10.1515/nanoph-2019-0483
18
[8] S.Y.Zhuo, Z.G.Zhao, Z. X. Xie, Y.F. Hao, Y.C. Xu, T.Y.Zhao, H.J. Li, E.M. Knubben, L.
Wen, L. Jiang, M.J. Liu, Complex multiphase organo hydrogels with programmable mechanics
toward adaptive soft-matter machines Sci. Adv. 2020, 6(5), eaax1464
[9] M.Wang, Y.J Chen, ,R. Khanc,H. Z. Liu, ,C. Chen,T. Chen, R.J Zhang, H. Li, A fast self-
healing and conductive nanocomposite hydrogel as soft strain sensor. Coll. Surf. A. 2019, 567,
139-149
[10]Y. Yamamoto, K. Kanao, T.Arie, S. Akita, K. Takei, Air Ambient-Operated pNIPAM-
Based Flexible Actuators Stimulated by Human Body Temperature and Sunlight. ACS Appl.
Mater. Inter. 2015, 7(20),11002-11006
[11] Z. Zhao, X. Kuang, C. Yuan, H. Jerry Qi, D.N. Fang, Hydrophilic/Hydrophobic Composite
Shape-Shifting Structures, ACS Appl. Mater. Interfaces. 2018, 10(23), 19932-19939
[12] T.J. Wallin, J. Pikul, R.F. Shepherd, 3D printing of soft robotic systems, Nat. Rev. Mat.
2018, 3, 84-100
[13]J. Koffler, W. Zhu, X. Qu, O. Platoshyn, J.N. Dulin, J. Brock, L. Graham, P. Lu, J.
Sakamoto, M. Marsala, S.C. Chen, M. H. Tuszynski, Putting 3D Printing to Work to Heal
Spinal Cord Injury, Nat. Med. 2019, 25, 263-269.
[14] M. Malinauskas. Ultrafast laser processing of materials: from science to industry. Light-
Sci. Appl. 2016, 5, e16133
[15] J. Xing, M. Zheng, X. Duan, Two-photon polymerization microfabrication of hydrogels:
an advanced 3D printing technology for tissue engineering and drug delivery, Chem. Soc. Rev.
2015, 44(15), 5031-5039
[16] S. Kawata, H.B. Sun, T. Tanaka, K. Takada. Finer features for functional microdevices.
Nat. 2001, 412(6848), 697-698
19
[17] Y.L. Zhang, Y.Tian, H. Wang, Z.C. Ma, D.D. Han, L. G. Niu, Q.D. Chen, H.B. Sun, Dual-
3D Femtosecond Laser Nanofabrication Enables Dynamic Actuation, Acs nano, 2019, 13(4),
4041-4048
[18] Y. Wu, X. Hao, R. Xiao, J. Lin, Z. L. Wu, J. Yin, J. Qian, Controllable bending of bi-
hydrogel strips with differential swelling. Acta.Mech. Solida. Sin. 2019, doi:10.1007/s10338-
019-00106-6
[19] D.D.Jin, Q.Y. Chen, T.Huang, J.Y. Huang, L. Zhang, H.L.Duan, Four-dimensional direct
laser writing of reconfigurable compound micromachines. Mater.Today. 2020, 32, 19-25
[20] A.A. Bauhofer, Harnessing Photochemical Shrinkage in Direct Laser Writing for Shape
Morphing of Polymer Sheets. Adv. Mater. 2017, 29,1703024
[21] X.Ning, X. Yu, Mechanically active materials in three-dimensional mesostructures. Sci.
Adv.2018, 4,eaat8313
[22] D. Karalekas, A. Aggelopoulos, Study of shrinkage strains in a stereolithography cured
acrylic photopolyrner resin. J. Mater. Proc. Tech. 2003, 136(1-3),146-150
[23]H. L. Sun, H. A. Klok, Z. Y. Zhong, Polymers from Nature and for Nature,
Biomacromolecules, 2018, 19, 1697−1700
[24] Y. Zhang, X. Le, Y. Jian, L.Wei, J.Zhang, T. Chen, 3D Fluorescent Hydrogel Origami for
Multistage Datan Security Protection, Adv.Funct. Mat. 2019, 29(46):1905514
[25] A. C. Almeida, J. Canejo, S. Fernandes, C. Echeverria, P. Almeida, M. Godinho, Cellulose-
Based Biomimetics and Their Applications, Adv. Mater. 2018, 30, 1703655
[26]A.Urrios, C. Parra-Cabrera, N. Bhattacharjee, A.M. Gonzalez-Suarez, L.G. Rigat-
Brugarolas, 3D-printing of transparent bio-microfluidic devices in PEG-DA, Lab Chip, 2016,
16, 2287–2294
20
[27] A.Tudor, C.Delaney, H. Zhang, Fabrication of soft, stimulus-responsive structures with
sub-micron resolution via two-photon polymerization of poly(ionic liquid)s, Mater. Today.
2018, 21(8), 807-816
[28] L. Wolski, M. Ziolek, Insight into pathways of methylene blue degradation with H2O2 over
mono and bimetallic Nb, Zn oxides. Appl. Cata. B-Enviro. 2018, 224, 634-647
[29] M. A. Tasdelen, V. Kumbaraci, S. Jockusch, N.J. Turro, N. Talinli, Photoacid generation
by stepwise two-photon absorption: Photoinitiated cationic polymerization of cyclohexene
oxide by using benzodioxinone in the presense of iodonium salt. Macromolecules. 2008, 41,
295-297
[30] Z. Moosavi-Tekyeh, N. Dastani, Intramolecular hydrogen bonding in N-
salicylideneaniline: FT-IR spectrum and quantum chemical calculations. J. Mol. Struct. 2015,
1102, 314-322
[31] Y. Liu, B. Shaw, M.D. Dickey, J. Genzer, Sequential self-folding of polymer sheets. Sci.
Adv. 2017,3, e1602417
[32] H. Liang, L. Mahadevan, Growth, geometry, and mechanics of a blooming lily. Proc.
Natl.Acad. Sci. U.S.A. 2011, 108, 5516–5521
[33] J. Van Hoorick, Cross-Linkable Gelatins with Superior Mechanical Properties Through
Carboxylic Acid Modification: Increasing the Two-Photon Polymerization Potential.
Biomacromolecules. 2017, 18(10), 3260-3272.
[34]E.D.Lemma, Mechanical Properties Tunability of Three-Dimensional Polymeric Structures
in Two-Photon Lithography. IEEE. Trans. Nanotechnol. 2017, 16 (1) ,23-31
[35] S. Wei, W. Lu, X. Le, C. Ma, H. Lin, B. Wu, J. Zhang, P. Theato, T. Chen, Bioinspired
Synergistic Fluorescence-Color Switchable Polymeric Hydrogel Actuator, Angew. Chem. Int.
Ed. 2019, 58(45), doi:10.1002/anie.201908437.
21
[36] R. C. P. Verpaalen, M. P. Cunha, T. A. P. Engels, M. G. Debije, and A. P. H. J. Schenning,
Liquid Crystal Networks on Thermoplastics: Reprogrammable Photo-Responsive Actuators,
Angew. Chem. Int. Ed. 2020, 59, 2–7.
[37] B. Han, Y.L. Zhang, L. Zhu, Y. Li, Z.C. Ma, Y.Q. Liu, X.L. Zhang, X.W. Cao, Q.D. Chen,
C.W. Qiu, H.B. Sun, plasmonic-assisted graphene oxide artificial muscles, Adv. Mat. 2018, 31,
1806386
[38] R.C. Lan, J. Sun, C. Shen , R. Huang, Z.P. Zhang , L.Y. Zhang, L. Wang , H. Yang,
Near-infrared photodriven self-sustained oscillation of liquid crystalline network film with
predesignated polydopamine coating, Adv. Mater. 2020, 1906319.
[39] Y.F. Tao, C.Y.R. Wei, J. W. Liu, C.S. Deng, S.Cai, W. Xiong, Nanostructured electrically
conductive hydrogels via ultrafast laser processing and self-assembly Nanoscale. 2019, 11(18),
9176-9184
[40] H. Zhang, X. G. Guo, J. Wu, D. Fang, Y. H. Zhang. Soft mechanical metamaterials with
unusual swelling behavior and tunable stress-strain curves. Sci. Adv. 2018,4, eaar8535
[41] N.W. Tschoegl, W.G. Knauss, I. Emri, Poisson's Ratio in Linear Viscoelasticity-A Critical
Review. Mech. Time-Depend. Mater. 2002, 6 (1),3-51